The present invention relates to a star scanner used for detecting the attitude of a space vehicle that is in a spinning motion in space.
As shown in FIG. 1, a conventional star scanner installed in a space vehicle comprises optical system 11 for forming an image of a star, a photoelectric transducer consisting of light-blocking plate 12 having a V-shaped (or N-shaped) slit pattern formed therein, and photomultiplier tube 13 located behind plate 12, and signal processor 14 for fetching the star image detection data from a signal output from photomultiplier 13. More particularly, as shown in FIG. 2A, a star image formed by optical system 11 is moved by the spinning of the vehicle and crosses slits A1 and A2 of plate 12. In this case, the light passing through slits A1 and A2 is detected by photomultiplier 13 and is transduced as a pulse signal having amplitude is corresponding to its intensity, as shown in FIG. 2B. Since slits A1 and A2 constitute a letter V (or N), two pulses (three pulses in the case of an N-shaped slit pattern) per star image are generated by photomultiplier 13. Pulse separation T of these pulses is determined in accordance with the positions at which the star image crosses slits A1 and A2, i.e., the incident angle (angle of elevation) of the input light. Therefore, when amplitude Is and pulse separation T of the output pulses from photomultiplier 13 are measured by signal processor 14, the star can be identified and attitude of the vehicle can be determined.
However, the conventional star scanner uses a photomultiplier tube as an optical detector for its photoelectric transducer. Since the photomultiplier tube is a vacuum tube, it requires a high-voltage power supply of 1 to 2 kV, which leads to an increase in power consumption. In addition, since a photomultiplier tube is bulky and heavy, and has poor reliability, it poses many problems when it is installed on a satellite, whose maintenance is difficult to perform.
In contrast to this, another conventional star scanner uses, as an optical detector, a bar-type semiconductor photosensive element such as a photodiode. For example, two photodiodes are arranged to constitute a letter V (or N), in order to provide a small, lightweight, low-power optical detector. However, such a photodiode has a lower sensitivity than that of a photomultiplier tube and tends to be easily influenced by noise. Particularly, a 1/f noise component of the photodiode has a power spectrum as shown in FIG. 3. As seen from FIG. 3, the 1/f noise component is very large in the low-frequency band and is a major factor causing noise. For example, when a star image passes the photosensitive surface of the photodiode, the output current signal of the photodiode changes, as shown in FIG. 4. The power spectrum with respect to the frequency is as shown in FIG. 5 and major components thereof are concentrated in the low-frequency band. In this manner, in a conventional star scanner which uses a semiconductor photosensitive element, such as a photodiode, as an optical detector, the influence of the 1/f noise component is large, and only a star brighter than that of first magnitude can be detected.